Oil paint, one of the oldest forms of paint, is based upon a curable binder derived from vegetable oils, referred to as hardening oils, such as linseed. Such oils are esters formed by reaction of fatty acid with glycerol: the hydrocarbon tail of the fatty acid includes conjugated dienes which can undergo mutual cross-linking in the presence of oxygen, or other oxygen generators, such as peroxides.
Modern, synthetic versions of such oil paints are referred to as alkyd paints. The term alkyd originates from the AL of polyhydric ALcohols and the CID (modified to KYD) of the aCIDs used to react together to form a polyester. Hence, the terms alkyd and polyester are synonymous. In conventional usage, the term alkyd is usually used to refer to polyesters modified to include fatty acid chains from hardening oils as side groups to the polymer chain.
A typical alkyd resin may be prepared by heating a hardening oil (also referred to as drying oils or semi-drying oils in the prior art and in this specification) such as linseed oil with an acid anhydride and glycerol to obtain a polyester with unsaturated side groups (say C12 to C24 hydrocarbon chains) derived from the fatty acid groups of the oil.
For use in paints, varnish, ink or coating material, the alkyd resin may be either emulsified in water or in aqueous solution, or may be dissolved in an organic solvent. For the sake of brevity, the term “alkyd paints” is used in this specification to mean water-based or solvent-based paints, varnish, ink, coating material or the like, which are based upon an alkyd binder curable by oxidative cross-linking. Typically, ambient oxygen from the air will provide the oxidative cross-linking, but other oxide sources such as peroxides may be employed additionally or alternatively, as set out herein.
The drying of alkyd paints is understood to take place through two identifiable, distinct mechanisms. Firstly, evaporation of solvent from the composition leads to a high viscosity, tacky layer being formed. Secondly, oxidative cross-linking takes place between the unsaturated fatty acid moieties of the alkyd resin within the layer resulting in the hardening of the film. These two mechanisms may be referred to as physical drying and chemical drying (the latter also referred to as oxidative drying or curing in this specification) and may occur concurrently and/or sequentially. The chemical drying stage is an auto-oxidation process which typically proceeds by a free-radical chain reaction mechanism which can be described in terms of the stages of initiation, propagation and termination. Many alkyd paints includes a siccative agent to act as an initiator of this chemical drying stage.
Cobalt compounds, particularly cobalt soaps, are generally widely used as siccative agents (catalysts for initiation of chemical drying) for the oxidative drying of alkyd paints or paints based on other curable resins. In recent years, the toxic, and potentially carcinogenic, nature of cobalt, as well as its undesirability in the environment as a waste product, has led to moves in the alkyd paint industry away from cobalt-based siccative compounds and towards alternative siccative compounds. The International patent application publication WO2008/003652 A1 discloses siccative compounds for air-drying of alkyd-based resins and coatings such as paint, varnish or wood stain inks and linoleum floor coverings, based on an iron/manganese complex containing tetradentate, pentadentate or hexadentate nitrogen donor ligands.
Binders curable by oxidative cross-linking, in addition to their use in paints, may also be employed as the basis for thermosetting resins used for a variety of industrial purposes such as casting materials, fibre reinforced materials and coatings.
When curable binders are for use in structural materials, rather than as paints, varnishes, inks or the like, curing may be achieved by means of the further incorporation of an unsaturated vinyl monomer, such as styrene, into the resin, to facilitate cross-linking (curing) of the resin in combination with a peroxide-type initiator through a free-radical copolymerisation mechanism. Once again, it is typical to use cobalt-based compounds as siccative agents (primary accelerator) for speeding the curing of such curable alkyd resins. As was set out above in relation to alkyd paints, it is desirable to replace cobalt-based siccative compounds with safer and more environmentally acceptable alternatives. International patent application publication WO2011/083309 A1 discloses accelerators for assisting in the curing of unsaturated polyester resins, vinyl ester resins and acrylic resins in conjunction with peroxide type initiators. The disclosed accelerators are based on iron/manganese complexes of a tridentate, tetradentate, pentadentate or hexadentate nitrogen donor ligand.
Throughout this specification, the terms “drying” and curing” are used interchangeably to refer to the transformation of a curable liquid composition from a liquid state into a solid state with a hard, solid surface.